1. Field of the Invention
The current invention relates to selective cooling, or hypothermia, of an organ, such as the brain, by cooling the blood flowing into the organ. This cooling can protect the tissue from injury caused by anoxia or trauma.
2. Background Information
Organs of the human body, such as the brain, kidney, and heart, are maintained at a constant temperature of approximately 37.degree. C. Cooling of organs below 35.degree. C. is known to provide cellular protection from anoxic damage caused by a disruption of blood supply, or by trauma. Cooling can also reduce swelling associated with these injuries.
Hypothermia is currently utilized in medicine and is sometimes performed to protect the brain from injury. Cooling of the brain is generally accomplished through whole body cooling to create a condition of total body hypothermia in the range of 20.degree. to 30.degree. C. This cooling is accomplished by immersing the patient in ice, by using cooling blankets, or by cooling the blood flowing externally through a cardiopulmonary bypass machine. U.S. Pat. No. 3,425,419 to Dato and U.S. Pat. No. 5,486,208 to Ginsburg disclose catheters for cooling the blood to create total body hypothermia. However, they rely on circulating a cold fluid to produce cooling. This is unsuitable for selective organ hypothermia, because cooling of the entire catheter by the cold fluid on its way to the organ would ultimately result in non-selective, or total body, cooling.
Total body hypothermia to provide organ protection has a number of drawbacks. First, it creates cardiovascular problems, such as cardiac arrhythmias, reduced cardiac output, and increased systemic vascular resistance. These side effects can result in organ damage. These side effects are believed to be caused reflexively in response to the reduction in core body temperature. Second, total body hypothermia is difficult to administer. Immersing a patient in ice water clearly has its associated problems. Placement on cardiopulmonary bypass requires surgical intervention and specialists to operate the machine, and it is associated with a number of complications including bleeding and volume overload. Third, the time required to reduce the body temperature and the organ temperature is prolonged. Minimizing the time between injury and the onset of cooling has been shown to produce better clinical outcomes.
Some physicians have immersed the patient's head in ice to provide brain cooling. There are also cooling helmets, or head gear, to perform the same. This approach suffers from the problems of slow cool down and poor temperature control due to the temperature gradient that must be established externally to internally. It has also been shown that complications associated with total body cooling, such as arrhythmia and decreased cardiac output, can also be caused by cooling of the face and head only.
Selective organ hypothermia has been studied by Schwartz, et. al. Utilizing baboons, blood was circulated and cooled externally from the body via the femoral artery and returned to the body through the carotid artery. This study showed that the brain could be selectively cooled to temperatures of 20.degree. C. without reducing the temperature of the entire body. Subsequently, cardiovascular complications associated total body hypothermia did not occur. However, external circulation of the blood for cooling is not a practical approach for the treatment of humans. The risks of infection, bleeding, and fluid imbalance are great. Also, at least two arterial vessels must be punctured and cannulated. Further, percutaneous cannulation of the carotid artery is very difficult and potentially fatal, due to the associated arterial wall trauma. Also, this method could not be used to cool organs such as the kidneys, where the renal arteries cannot be directly cannulated percutaneously.
Selective organ hypothermia has also been attempted by perfusing the organ with a cold solution, such as saline or perflourocarbons. This is commonly done to protect the heart during heart surgery and is referred to as cardioplegia. This procedure has a number of drawbacks, including limited time of administration due to excessive volume accumulation, cost and inconvenience of maintaining the perfusate, and lack of effectiveness due to temperature dilution from the blood. Temperature dilution by the blood is a particular problem in high blood flow organs such as the brain. For cardioplegia, the blood flow to the heart is minimized, and therefore this effect is minimized.
Intravascular, selective organ hypothermia, created by cooling the blood flowing into the organ, is the ideal method. First, because only the target organ is cooled, complications associated with total body hypothermia are avoided. Second, because the blood is cooled intravascularly, or in situ, problems associated with external circulation of blood are eliminated. Third, only a single puncture and arterial vessel cannulation is required, and it can be performed at an easily accessible artery such as the femoral, subclavian, or brachial. Fourth, cold perfusate solutions are not required, thus eliminating problems with excessive fluid accumulation. This also eliminates the time, cost, and handling issues associated with providing and maintaining cold perfusate solution. Fifth, rapid cooling can be achieved. Sixth, precise temperature control is possible.
Previous inventors have disclosed the circulation of a cold fluid to produce total body hypothermia, by placing a probe into a major vessel of the body. This approach is entirely unfeasible when considering selective organ hypothermia, as will be demonstrated below.
The important factor related to catheter development for selective organ hypothermia is the small size of the typical feeding artery, and the need to prevent a significant reduction in blood flow when the catheter is placed in the artery. A significant reduction in blood flow would result in ischemic organ damage. While the diameter of the major vessels of the body, such as the vena cava and aorta, are as large as 15 to 20 mm., the diameter of the feeding artery of an organ is typically only 4.0 to 8.0 mm. Thus, a catheter residing in one of these arteries cannot be much larger than 2.0 to 3.0 mm. in outside diameter. It is not practical to construct a selective organ hypothermia catheter of this small size using the circulation of cold water or other fluid. Using the brain as an example, this point will be illustrated.
The brain typically has a blood flow rate of approximately 500 to 750 cc/min. Two carotid arteries feed this blood supply to the brain. The internal carotid is a small diameter artery that branches off of the common carotid near the angle of the jaw. To cool the brain, it is important to place some of the cooling portion of the catheter into the internal carotid artery, so as to minimize cooling of the face via the external carotid, since face cooling can result in complications, as discussed above. It would be desirable to cool the blood in this artery down to 32.degree. C., to achieve the desired cooling of the brain. To cool the blood in this artery by a 5 C.degree. drop, from 37.degree. C. down to 32.degree. C., requires between 100 and 150 watts of refrigeration power.
In order to reach the internal carotid artery from a femoral insertion point, an overall catheter length of approximately 100 cm. would be required. To avoid undue blockage of the blood flow, the outside diameter of the catheter can not exceed approximately 2 mm. Assuming a coaxial construction, this limitation in diameter would dictate an internal supply tube of about 0.70 mm. diameter, with return flow being between the internal tube and the external tube.
A catheter based on the circulation of water or saline operates on the principle of transferring heat from the blood to raise the temperature of the water. Rather than absorbing heat by boiling at a constant temperature like a freon, water must warm up to absorb heat and produce cooling. Water flowing at the rate of 5.0 grams/sec, at an initial temperature of 0.degree. C. and warming up to 5.degree. C., can absorb 100 watts of heat. Thus, the outer surface of the heat transfer element could only be maintained at 5.degree. C., instead of 0.degree. C. This will require the heat transfer element to have a surface area of approximately 1225 mm.sup.2. If a catheter of approximately 2.0 mm. diameter is assumed, the length of the heat transfer element would have to be approximately 20 cm.
In actuality, because of the overall length of the catheter, the water would undoubtedly warm up before it reached the heat transfer element, and provision of 0.degree. C. water at the heat transfer element would be impossible. Circulating a cold liquid would cause cooling along the catheter body and could result in non-specific or total body hypothermia. Furthermore, to achieve this heat transfer rate, 5 grams/sec of water flow are required. To circulate water through a 100 cm. long, 0.70 mm. diameter supply tube at this rate produces a pressure drop of more than 3000 psi. This pressure exceeds the safety levels of many flexible medical grade plastic catheters. Further, it is doubtful whether a water pump that can generate these pressures and flow rates can be placed in an operating room.